Method of joining plastic optical fibers to each other

ABSTRACT

A splicing method for plastic optical fibers, wherein solvent splicing is carried out by means of an organic solvent 4 capable of dissolving or swelling the plastic material of plastic optical fibers 1 and 2.

TECHNICAL FIELD

The present invention relates to a splicing method for plastic opticalfibers themselves.

BACKGROUND ART

Heretofore, as a splicing method for optical fibers, with respect tooptical fibers employing silica as a material having a high meltingpoint, a method has been known in which a very high temperature iscreated by electric arc discharge, and optical fibers thereby melted arespliced to each other. However, to let very small portions like opticalfiber tips undergo electric discharge, an extremely high level oftechnique and precision in control are required, and to let silica bemelted, a large electric power was required.

On the other hand, with respect to plastic optical fibers such asacrylic plastic optical fibers or fluorine type plastic optical fibers(see e.g. JP-A-8-5848), no method for efficiently splicing opticalfibers themselves, has been known.

The present invention is intended to provide a method for readilysplicing plastic optical fibers themselves, whereby the splicingefficiency is increased, and an increase in the transmission loss isreduced.

DISCLOSURE OF THE INVENTION

The present invention is a splicing method for plastic optical fibers,which is a method for splicing ends of plastic optical fibersthemselves, characterized in that solvent splicing is carried out bymeans of an organic solvent capable of dissolving or swelling theplastic material of the plastic optical fibers.

In the present invention, a plastic optical fiber is "a general term foran optical fiber cord and an optical fiber string, of which at least alight transmitting portion is made of a plastic material"(hereinaftermay sometimes be referred to simply as an optical fiber), and an opticalfiber cord is one having an optical fiber string covered with a coveringmaterial. Here, the light-transmitting portion is meant for a coreportion in the case of a refractive index stepped optical fiber, or aportion where at least 5% of the maximum intensity occupies in thedistribution in the fiber radial direction of the intensity of lightoutgoing from the fiber in the case of a graded-index optical fiber.

In the present invention, solvent splicing means that plastic materialsthemselves dissolved or swelled by the organic solvent, are contacted ormixed, followed by evaporation of the organic solvent in the plasticmaterials, whereby the plastic materials themselves are solidified, sothat the plastic optical fibers themselves are spliced to each other.

In the present invention, the method for solvent-splicing the ends ofplastic optical fibers themselves, may, for example, be (1) a methodwherein the ends of optical fibers themselves are solvent-spliced bypermitting the solvent to penetrate into a clearance between the ends ofoptical fibers themselves, which is formed by bringing the ends ofoptical fibers themselves close to each other, (2) a method wherein theends of optical fibers themselves are solvent-spliced by contacting theoptical fiber ends themselves dissolved or swelled by the organicsolvent, or (3) a method wherein the ends of optical fibers themselvesare solvent-spliced, while permitting the optical fibers themselves tobutt against each other by the bending stress of the optical fibers.

The splicing method of the present invention makes it possible toreadily splice plastic optical fibers themselves and to increase thesplicing efficiency and reduce an increase of transmission loss.Further, by using a reinforcing member for the joint portion, positionsetting of the optical fibers can be facilitated, a proper pressure caneasily be exerted to the joint surfaces of the optical fibers, and thejoint portion of the plastic optical fibers themselves can bereinforced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrating optical fibers and a solvent.

FIG. 2 is a schematic view of a supporting tool for splicing opticalfibers, which has a V-groove and a void space.

FIG. 3 is a schematic plan view showing the state in which opticalfibers are placed on the supporting tool.

FIG. 4 is a schematic plan view showing the state in which opticalfibers are placed on a reinforcing member for the joint portion.

BEST MODE FOR CARRYING OUT THE INVENTION

Now, the present invention will be described with reference to FIGS. 1to 4. FIG. 1 is a schematic view showing optical fibers 1 and 2 to bespliced, and a solvent 4. FIG. 2(a) is a schematic plan view of asupporting tool 7 for splicing optical fibers, which has a V-groove 5and a void space 6, and FIG. 2(b) is a schematic front view of thesupporting tool 7. FIG. 3 is a schematic plan view showing the state inwhich optical fibers 1 and 2 are placed on the supporting tool 7. FIG. 4is a schematic plan view showing the state wherein optical fibers areplaced on a reinforcing member for the joint portion.

The joint end surfaces of forward ends of the optical fibers 1 and 2 maynot necessarily be completely flat surfaces and may be in a rough stateas simply cut by e.g. a razor. It is preferred that the optical fibers 1and 2 are butted against each other by means of a micro positionerapparatus such as a three axes micro positioner apparatus for opticalexperiments.

When the solvent 4 is applied to the butting portion 3 of the opticalfibers 1 and 2, the joint ends surfaces made of a plastic material ofthe forward ends of the respective optical fibers will be partiallymelted and mixed. Then, when the melted plastic material dries andsolidifies, the optical fibers 1 and 2 will be fixed.

Instead of using the micro positioner apparatus, the optical fibers 1and 2 may be placed on a supporting tool 7 for splicing optical fibers,which has a V-groove 5 and a void space 6, so that the optical fibers 1and 2 will be butted and spliced. In this case, there is a merit that anexcess solvent at the butting portion 3 can be discharged from the voidspace 6. The material for the supporting tool 7 is preferably stainlesssteel, aluminum alloy or brass.

In the method for solvent-splicing the ends of the optical fibersthemselves by permitting the optical fibers themselves to butt againsteach other by the bending stress of the optical fibers, it is preferredto employ a reinforcing member 13 for the joint portion.

The optical fibers 1 and 2 are inserted from both ends of thereinforcing member 13 for the joint portion to butt against each otherat the optical fiber-butting portion 3. The reinforcing member 13 forthe joint portion has optical fiber securing portions 8 and 9 and anoptical fiber guide 12. The reinforcing member 13 for the joint portionmay be integrally formed, or the optical fiber securing portions 8 and 9and the optical fiber guide 12 may preliminarily be formed, and they maybe spliced to form the reinforcing member 13 for the joint portion. Thematerial for the reinforcing member 13 for the joint portion ispreferably the same as the material for the supporting tool 7 or varioussynthetic resins.

The optical fiber guide 12 has alignment portions 10 and 11 and a voidspace 6. The alignment portions 10 and 11 are provided with grooveswhich are capable of precisely guiding and linearly sliding the opticalfibers 1 and 2.

At bending stress portions 14 and 15, spaces are secured so that theoptical fibers can be bent. By pushing the optical fibers from both endswith a proper force, the optical fibers will be arched at the bendingstress portions 14 and 15. Then, the optical fibers are secured at theoptical fiber securing portions 8 and 9, whereby a butting force willcontinuously be exerted to the optical fiber butting portion 3 by theforce of the arched optical fibers which try to regain the initialstate.

When a solvent is applied to the butting portion 3 from the void space 6at the butting portion, the cut surfaces of the respective opticalfibers will be partially dissolved and mingled. At that time, theoptical fibers will be shortened for a length corresponding to thedissolved amount, but by the stress of the arched optical fibers, aconstant butting force can continuously be exerted. Thereafter, when thedissolved resin dries and solidifies, the optical fibers 1 and 2 will befixed. The void space 6 makes the application of the solvent possibleand at the same time serves to promote drying of the solvent.

As the method for securing the optical fibers at the securing portions 8and 9, a method of employing an adhesive or a method of mechanicallysecuring may, for example, be mentioned. After splicing the opticalfibers 1 and 2, the optical fibers may be secured at the alignmentportions 10 and 11. As such a securing method, the same method as themethod for securing optical fibers at the securing portions 8 and 9 maybe employed.

Further, the optical fibers 1 and 2 may be secured at the securingportions 8 and 9 or at the alignment portions 10 and 11, and the opticalfibers 1 and 2 may be butted without splicing at the butting portion 3.

The arching of the optical fibers at the bending stress portions 14 and15, also serves to cancel out the difference in linear expansion by heatbetween the reinforcing member 13 for the joint portion and the opticalfibers 1 and 2.

The optical fiber diameter is preferably from 100 to 1000 μm, morepreferably from 250 to 750 μm. The length in the optical fiber directionof the optical fiber guide 12 is preferably from 10 to 100 times, morepreferably from 20 to 50 times, the optical fiber diameter. The lengthin the optical fiber direction of the stress portions 14 and 15 ispreferably from 10 to 50 times, more preferably from 20 to 40 times, theoptical fiber diameter.

When the solvent 4 is applied, if the optical fiber 1 and the opticalfiber 2 are too close, the solvent 4 may not well penetrate to the jointsurface of the optical fibers 1 and 2. In such a case, the optical fiber1 and the optical fiber 2 may be separated a little. The distance forsuch separation is preferably from 1 to 50 μm, more preferably from 1 to30 μm.

Otherwise, after permitting the solvent to penetrate, the optical fibersmay be brought to be close to each other. Further, upon drying, theplastic material shrinks a little, and the optical fibers may be broughtto be close to each other taking such shrinkage into account.

The viscosity of the solvent is preferably such that it will penetrateinto the clearance between the optical fibers 1 and 2 and at the sametime will not spread so much. Therefore, the viscosity of the solventmay be adjusted by dissolving the plastic material of the optical fibersin the solvent. The concentration of the plastic material in the solventsolution is preferably from 0.01 to 30 wt %, more preferably from 0.1 to10 wt %.

Further, as a method for drying the solvent, not only natural drying,but also conduction heat, radiation heat or heat by high frequencyinduction may, for example, be employed. Further, in the method ofbutting the optical fibers, micro positioner may be carried out underobservation by e.g. a microscope, or micro positioner may be carried outby propagating light and measuring the amount.

The solvent to be used in the present invention, may be suitablyselected among those which are capable of dissolving or swelling theplastic material for the joint end surfaces of the forward ends of theoptical fibers. In the case of a fluorine type plastic material, afluorine type organic solvent, particularly a perfluoro organic solvent,is preferred. The perfluoro organic solvent may, for example, be aperfluoroalkane such as perfluorohexane, perfluorooctane orperfluorodecane, a perfluorotrialkylamine such asperfluorotripropylamine or perfluorotributylamine, a perfluoro cyclicether such as perfluoro(2-butyltetrahydrofuran), or a perfluoro aromaticcompound such as perfluorobenzene.

At the time of splicing optical fibers constituted by different types ofplastic materials, such as the string and the covering material of anoptical fiber cord, or the core and the cladding of an optical fiberstring, a solvent capable of commonly dissolving the different types ofplastic materials, may be employed, or two or more solvents may be useddepending upon the types of the plastic materials.

The optical fibers in the present invention may be step-index opticalfibers or graded-index optical fibers. Further, they may be thosewherein the light transmitting portion is made of a non-fluorine typeplastic material, or a fluorine type plastic material, or they may be acombination of a non-fluorine type plastic material and a fluorine typeplastic material, such as a combination of a core made of a non-fluorinetype plastic material and a cladding made of a fluorine type plasticmaterial.

As the fluorine type plastic material, a noncrystallinefluorine-containing polymer having substantially no C--H bond ispreferred. More preferred is a fluorine-containing polymer which isnon-crystalline with substantially no C--H bond and which has a ringstructure in its main chain.

As the above fluorine-containing polymer having a cyclic structure inits main chain, a fluorine polymer having a fluorine-containingalicyclic structure, a fluorine-containing imide ring structure, afluorine-containing triazine ring structure or a fluorine-containingaromatic ring structure, is preferred. As the fluorine-containingpolymer having a fluorine-containing alicyclic structure, one having afluorine-containing aliphatic ether ring is more preferred.

The fluorine-containing polymer having a cyclic structure in its mainchain preferably has at least 20 mol %, more preferably at least 40 mol%, of polymer units having a ring structure, from the viewpoint oftransparency, mechanical properties, etc.

The fluorine-containing polymer having a fluorine-containing aliphaticring structure is a preferred polymer for such a reason that as comparedwith a fluorine-containing polymer having a fluorine-containing imidering structure, a fluorine-containing triazine ring structure or afluorine-containing aromatic ring structure, the polymer molecules areless likely to be aligned even at the time of hot stretching or forminginto optical fibers by melt spinning, and consequently, it is free fromscattering of light.

Preferred as the polymer having a fluorine-containing aliphatic ringstructure is one obtainable by polymerizing a monomer having afluorine-containing cyclic structure, or a polymer having afluorine-containing alicyclic structure in its main chain obtainable bycyclic polymerization of a fluorine-containing monomer having at leasttwo polymerizable double bonds.

The polymer having a fluorine-containing alicyclic ring structure in itsmain chain, obtainable by polymerizing a monomer having afluorine-containing alicyclic ring structure, is known, for example, byJP-B-63-18964. Namely, a polymer having a fluorine-containing alicyclicring structure in its main chain, can be obtained by homopolymerizationof a monomer having a fluorine-containing alicyclic ring structure, suchas perfluoro(2,2-dimethyl-1,3-dioxole), or by copolymerizing thismonomer with a radical polymerizable monomer such astetrafluoroethylene, chlorotrifluoroethylene or perfluoro(methyl vinylether).

Further, the polymer having a fluorine-containing aliphatic ringstructure in its main chain, obtainable by cyclic polymerization of afluorine-containing monomer having at least two polymerizable doublebonds, is known, for example, by JP-A-63-238111 or JP-A-63-238115.Namely, a polymer having a fluorine-containing aliphatic ring structurein its main chain, can be obtained by cyclic polymerization of a monomersuch as perfluoro(allyl vinyl ether) or perfluoro(butenyl vinyl ether),or by copolymerizing such a monomer with a radical polymerizable monomersuch as tetrafluoroethylene, chlorotrifluoroethylene, orperfluoro(methyl vinyl ether).

Further, a polymer having a fluorine-containing aliphatic cyclicstructure in its main chain may also be obtained by copolymerizing amonomer having a fluorine-containing aliphatic ring structure such asperfluoro(2,2-dimethyl-1,3-dioxole) with a fluorine-containing monomerhaving at least two polymerizable double bonds such as perfluoro(allylvinyl ether) or perfluoro(butenyl vinyl ether).

As a step-index optical fiber having a fluorine type plastic material asthe core material, one known, for example, by JP-A-4-189862, may bementioned.

As the above-described graded-index optical fiber, one made of a matrixresin having a refractive index difference and a diffusate, wherein thediffusate is distributed in the matrix resin with a concentrationgradient along a certain specific direction. Particularly preferred is agraded-index fluorine type plastic optical fiber comprising afluorine-containing polymer as a matrix resin and a low molecular weightfluorine type compound as a diffusate, since it has a low transmissionloss and a high transmission zone within a wide range of transmissionzone (see JP-A-8-5848).

In such a case, the number average molecular weight of thefluorine-containing polymer is preferably from 10,000 to 5,000,000, morepreferably from 50,000 to 1,000,000. The number average molecular weightof the low molecular weight fluorine type compound is preferably from300 to 10,000, more preferably from 300 to 5,000.

Now, the present invention will be described in further detail withreference to Examples. However, it should be understood that the presentinvention is by no means restricted by such specific Examples.

EXAMPLE 1

(Preparation Example)

35 g of perfluoro(butenyl vinyl ether) (PBVE), 150 g of deionized waterand 90 mg of ((CH₃)₂ CHOCOO)₂ as a polymerization initiator, were putinto an autoclave made of pressure resistant glass having an internalcapacity of 200 ml. The interior of the system was replaced three timeswith nitrogen, and then suspension polymerization was carried out at 40°C. for 22 hours. As a result, a polymer having a number averagemolecular weight of about 1.5×10⁵ (hereinafter referred to as polymer A)was obtained in an amount of 28 g.

The specific viscosity [η] of polymer A was 0.50 dl/g at 30° C. inperfluoro(2-butyltetrahydrofuran) (PBTHF) Polymer A had a glasstransition point of 108° C. and was a tough transparent glassy polymerat room temperature. Further, its 10% thermal decomposition temperaturewas 465° C., and the solubility parameter was 5.3 (cal/cm³)^(1/2), andthe refractive index was 1.34.

EXAMPLE 2

(Working Example)

Polymer A obtained in Example 1 was dissolved in a PBTHF solvent, and1,3-dibromotetrafluorobenzene (DBTFB) having a refractive index of 1.52and a difference in the solubility parameter from polymer A of 3.2(cal/cm³)^(1/2), was added thereto in an amount of 12 wt % to obtain amixed solution. This solution was subjected to solvent removal to obtaina transparent mixed polymer (hereinafter referred to as polymer B).

Polymer A was melted, and while injecting a molten liquid of polymer Bat the center, melt spinning was carried out at 300° C. to obtain agraded-index optical fiber (fiber diameter: 350 μm) wherein therefractive index gradually lowered from the center towards theperiphery.

This graded-index optical fiber was cut by a razor, then placed on theV-groove having a three axes micro positioner apparatus for opticalexperiments, and a weight was placed thereon to secure it. Then, whileobserving the joint portion by a magnifier, position setting was carriedout by the micro positioner apparatus so that the cut surfaces cameclose to each other with a distance of 10 μm.

Then, at the joint portion, PBTHF was dropped by a syringe, whereuponPBTHF penetrated between the two optical fibers. They were left to standabout 30 minutes, and then removed from the V-groove, whereby theoptical fibers were found to be spliced. The coupling loss was measuredby injected light into the optical fibers and was found to be 0.26 dB.

EXAMPLE 3

(Working Example)

The optical fibers 1 and 2 of Example 2 cut by a razor were insertedfrom both ends of the reinforcing member 13 for the joint portion inFIG. 4 to butt against each other at the optical fiber butting portion3. Further, by pushing the optical fibers with a proper force from bothends, the optical fibers were slightly arched at the bending stressportions 14 and 15. In that state, an adhesive was injected to theoptical fiber securing portions 8 and 9, and the optical fibers 1 and 2and the reinforcing member 13 for the joint portion were held until theywere fixed.

Then, from the void space 6 of the butting portion, a PBTHF solution ofPBVE (concentration: 2 wt %) was applied to the butting portion 3, therespective optical fiber cutting surfaces were partially dissolved andmingled, and then, when the dissolved resin dried and solidified, theoptical fibers 1 and 2 were fixed. The coupling loss was measured byinjected light into the optical fibers and was found to be 0.5 dB.

What is claimed is:
 1. A splicing method for plastic optical fibers,which is a method for splicing ends of plastic optical fibersthemselves, characterized in that solvent splicing is carried out bymeans of an organic solvent capable of dissolving or swelling theplastic material of the plastic optical fibers.
 2. The splicing methodaccording to claim 1, wherein the plastic optical fibers are fluorinetype plastic optical fibers, and the organic solvent is a fluorine typeorganic solvent.
 3. The splicing method according to claim 1, whereinthe plastic material is a non-crystalline fluorine-containing polymerhaving substantially no C--H bond.
 4. The splicing method according toclaim 1, wherein the fluorine type organic solvent is a perfluoroorganic solvent.
 5. The splicing method according to claim 1, whereinthe plastic optical fibers are graded-index optical fibers.
 6. Thesplicing method according to claim 1, wherein the solvent splicing isone wherein the ends of optical fibers themselves are solvent-spliced bypermitting the solvent to penetrate into a clearance between the ends ofoptical fibers themselves, which is formed by bringing the ends ofoptical fibers themselves close to each other.
 7. The splicing methodaccording to claim 1, wherein the solvent splicing is one wherein theends of optical fibers themselves are solvent-spliced by contacting theends of optical fibers themselves dissolved or swelled by the organicsolvent.
 8. The splicing method according to claim 1, wherein thesolvent splicing is one wherein the ends of optical fibers themselvesare solvent-spliced while permitting the optical fibers themselves tobutt against each other by the bending stress of the optical fibers. 9.The splicing method according to claim 1, wherein the organic solventcontains a plastic material.